The present invention relates to a metallizing composition suitable for use with electronic parts such as ceramic packages, ceramic multi-layered substrates, transistor packages, rectifier containers, ceramic tubes for vacuum circuit breakers and terminal bushings that employ low-temperature firing ceramics and noble-metal based metallizing composition, either alone or in combination with Cu-based metallizing composition.
With the recent tendency in the IC (integrated circuit) industry to increase the packing density of IC packages, multi-layered circuit boards, etc., to a very high level, a demand has arisen for commercialization of low-dielectric constant ceramics and low-resistance conductors.
It has been known in this field of art to use metallizing compositions of noble metals including Au, Au-Pt alloy, Ag-Pt alloy and Ag-Pd alloy, and they have the advantage of being firable in air atmosphere. On the other hand, however, the metallizing composition of these noble metals is costly.
In contrast, Cu-based metallizing composition is comparatively inexpensive and has recently come to be used in ceramic circuit boards by virtue of the added advantage of high electroconductivity. The major problem, however, with Cu-based metallizing composition is that it is very difficult to achieve complete combustion of the organic components in the metallizing composition without oxidizing Cu. If Cu is oxidized to form CuO, the conductivity and solderability of the metallizing composition are reduced. If some measure is taken to prevent the formation of CuO, the organic components will remain partly unburnt. The measure for preventing the formation of CuO is described, for example, in JP-A-55-128899 (The term "JP-A" as used herein means an "unexamined published Japanese patent application".), wherein a firing atmosphere is a neutral or reducing one, because the firing in air atmosphere makes Cu oxidize to form CuO according to a reaction formula: 2Cu + O.sub.2 .fwdarw. CuO.
Under these circumstances, the usual practice taken for noble-metal based metallizing composition is to simply fire it in air atmosphere, whereas Cu-based metallizing composition is calcined in air atmosphere at 500.degree. to 700.degree. C. so as to burn away the resin content, and thereafter, CuO that has been formed by oxidation is converted to Cu by treatment in a reducing atmosphere at 400.degree. to 600.degree. C., followed by final firing in non-oxidizing atmosphere. Therefore, in view of the differences in firing conditions between noble-metal based metallizing compositions and Cu-based metallizing compositions, it has been theoretically impossible to use the two types of metallizing compositions simultaneously in the same electronic part (or product).
As another problem, commercial noble-metal (e.g., Au) based metallizing composition typically contains lead borosilicate glass, so if it is fired in a non-oxidizing atmosphere, PbO in the glass is reduced to a metallic lead. Since the metallic lead causes such adverse effects as deterioration of the other glass components, the desired adhesion to a substrate cannot be sufficiently retained to justify the use of the noble-metal based metallizing composition in commercial applications.
Cu-based metallized packages are currently under development and Cu-based wire bonding techniques and die attach (D/A) techniques are not yet to be established. Under this situation, there is a recognized need for applying noble-metal based metallizing composition to those areas where bonding or D/A is to be performed.